Allow ChoiceField and friends to take a callable for choices kwarg

[Jeremy Dunck]

Sometimes we'd like choices to be dynamic (often the result of an ORM query), but because models are declarative, this forces queries to run with Django partially loaded.

It would be good to be able to make choices a callable so that these queries could be deferred until Django is fully loaded.

I'm attaching a rough patch; if this passes design-decision, I can polish it up with tests.

[Jannis Leidel]

Yeah, seems reasonable, although I'd prefer something like what the model fields do, taking any iterable instead:
​http://code.djangoproject.com/browser/django/trunk/django/db/models/fields/__init__.py?rev=15982#L393

Would that solve the issue for you, too?

[Jeremy Dunck]

Hmm, writing a generator to do a query only on the first execution of the generator is a little fiddly. (If there's a nice way to write it, please give an example.)

There's a common pattern of accepting callables - maybe we should add callable to model choices, too?

[Julien Phalip]

This was actually already suggested in #13181.